Multiple feature master cylinder

ABSTRACT

A brake failure warning switch, a brake proportioning valve with bypass and a metering valve are provided in a single master cylinder housing. During normal brake application, the proportioning valve reduces the braking effort of the rear wheels compared to that of the front wheels. The warning device and proportioning valve are interconnected in such a manner that a failure in the front brake hydraulic system not only actuates a brake failure indicator but also holds the proportioning valve open to permit the remaining rear system to deliver its maximum braking capability. The metering valve holds off hydraulic pressure to front wheel disc brakes until sufficient pressure has built up in the rear system to engage the shoes with the brake drum, thus energizing the disc brakes at approximately the same time as the rear wheel drum brakes.

United States Patent Shutt Aug. 29, 1972 154] MULTIPLE FEATURE MASTERCYLINDER [72] Inventor: Paul B. Shutt, St. Joseph, Mich.

[73] Assignee: The Bendix Corporation [22] Filed: March 2, 1971 [21]Appl. No.: 120,145

[52] U.S. Cl. ..60/54.5 E, 188/349, 200/82 D [51] Int. Cl ..Fl5b 7/00,HOlh 35/38 [58] Field of Search ..60/54.5 E, 54.6 E; 303/6 C; 188/349;200/82 D [56] References Cited UNITED STATES PATENTS 3,522,706 8/1970Bueler ..60/54.5 E 3,556,607 1/1971 Shutt et al. ..60/54.5 E 3,462,2008/1969 Lewis et al. ..60/54.5 E 3,321,914 5/1967 MacDufi ..188/3493,416,315 12/1968 Wortz ..60/54.6 E 3,480,333 11/1969 Stelzer ..303/6 C3,597,008 8/ 1971 Falk ..60/54.5 E 3,586,384 6/1971 Falk ..60/54.5 E

3,597,015 8/1971 Stokes ..60/54.5E

Primary Examiner-Martin P. Schwadron Assistant Examiner-A. M. ZupcicAttorney-William N. Antonis and Plante, Hartz,

Smith & Thompson ABSTRACT A brake failure warning switch, a brakeproportioning valve with bypass and a metering valve are provided in asingle master cylinder housing. During normal brake application, theproportioning valve reduces the braking effort of the rear wheelscompared to that of the front wheels. The warning device andproportioning valve are interconnected in such a manner that a failurein the front brake hydraulic system not only actuates a brake failureindicator but also holds the proportioning valve open to permit theremaining rear system to deliver its maximum braking capability. Themetering valve holds off hydraulic pressure to front wheel disc brakesuntil sufficient pressure has built up in the rear system to engage theshoes with the brake drum, thus energizing the disc brakes atapproximately the same time as the rear wheel drum brakes.

14 Claims, 5 Drawing Figures PATENTEDmszs m2 SHEET 2 OF 2 JNVENTOR PAULB. SHUT 7- 1] 'TTOPNEYS MULTIPLE FEATURE MASTER CYLINDER BACKGROUND OFTHE INVENTION This invention relates to a novel multi-feature mastercylinder for use in a dual hydraulic vehicular braking system.

In a vehicle having a split system master cylinder, two separate fluidsystems connect the master cylinder to a respective set of front andrear brakes, the object being to simultaneously actuate them. Inpractice, however, where disc brakes are used on front wheels and drumbrakes on the rear, this simultaneity of operation does not occur. Thefront wheel disc brakes require very little initial actuating pressurewhereas the rear drum brakes require substantial initial actuatingpressure from the master cylinder to move the brake shoes intoengagement with the drum. It is, therefore, desirable to provide amaster cylinder for a split hydraulic vehicular braking system whichembodies as a component part a metering valve to effect the pressuredelay required for desired disc brake operation.

Also, as deceleration magnitude caused by braking increases, it isreadily apparent to one skilled in the art that a vehicular weight shiftoccurs from one axle to the other. The weight shift during forwardtravel from the rear to the front wheels would be proportional to thedeceleration rate caused by the brake application in tensity This shiftdictates that proportionately less braking effortbe exerted through therear wheels thus preventing rear wheel lock-up and resultant skiddingand possible loss of control over the vehicle. In furtherance of thispurpose, it is therefore desirable to furnish a master cylinder for asplit hydraulic vehicular braking system which includes as a componentpart a proportioning valve which not only provides a proportionatepressure decrease in the rear braking system to prevent wheel slide asbrake application intensity during forward travel increases, but alsofunctions to bypass proportioning effects of the valve should pressurebe lost in the front braking system thus givingthe maximum brakingeffect from the remaining rear braking system.

Additionally, where the braking master cylinder is of the split systemtype a malfunction in either system resulting in loss of hydraulicpressure and/or fluid loss and the attendant loss of that system brakingcapability could go unnoticed by the vehicle operator during routinebraking applications, but lead to disastrous consequences in anemergency when total braking capability becomes mandatory.

Existing brake system failure warning devices generally function toindicate the failure only while the brakes are applied. Even if one ofthe systems has failed completely, the vehicle operator is apprised ofthis fact only during a brake application. Or, if the failure indicatorremains activated upon release of the brakes, it also persists evenafter the defective brake system has been repaired and pressure restoreduntil the warning device is reset by a mechanic.

It is, therefore, desirable to provide a vehicular hydraulic brakingsystem split master cylinder containing a hydraulic system failurewarning device that, once actuated by a failure in the hydraulic system,continuously warns the driver of the existing failure until thedefective system is repaired. At this time, the device shouldautomatically reset itself.

SUMMARY OF THE INVENTION It is an object of this invention to provide adual master cylinder for use in a dual hydraulic vehicular brakingsystem which is capable of metering hydraulic fluid pressure to frontwheel disc brakes so that they are not actuated prior to actuation ofthe rear drum brakes.

Another object of the invention is to provide a dual master cylinder foruse in a dual hydraulic vehicular braking system which proportions brakesystem pressure to the rear wheel brakes depending upon the intensity ofthe braking application and thus the ratio of the weight transferredfrom the rear of the front wheel brakes of the vehicle so that prematurerear wheel skid is avoided.

A further object of the invention is to provide acomponent proportioningvalve feature which is bypassed in the event of malfunction caused byhydraulic fluid loss or other cause of hydraulic fluid pressure loss inthe front wheel brakes so that the capability of the remaining rearbrake system is fully utilized to aid in stopping the vehicle until themalfunctioning system can be repaired. The bypass mechanism is thenautomatically reset upon repair of the system to re-establish theproportioning effect.

A still further object is to provide a dual master cylinder embodying abrake system failure warning. switch which is capable of indicating anypredetermined excessive pressure differential between the two hydraulicbrake systems resulting from fluid loss or pressure loss in one of thesystems.

It is an object of this invention to provide brake failure warningindication which will be maintained continuously until normal brakeapplication fluid pressure is restored to both systems, at which timethe warning device automatically resets itself to the normal,non-actuated position with the first brake application subsequent to therepair without disassembly of the master cylinder or bleeding of thebrake lines.

Another object of this invention is to provide an integral multi-featuremaster cylinder which is much less expensive to manufacture thanindividual components accomplishing the same result and wherein thecomponents of the multi-feature master cylinder are all included in asingle housing, thus conserving space and grouping all components at onecentral location.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of my mastercylinder schematically connected with conventional front disc brakes andrear drum brakes of an automobile;

FIG. 2 illustrates an enlarged isometric view of the rear brake systempiston shown removed from FIG. 1;

FIG. 3 illustrates an enlarged isometric view of the brake failureswitch expanding sleeve actuating element shown removed from FIG. 1;

FIG. 4 depicts an alternate construction method for the expanding sleeveactuating element of FIG. 3; and

FIG. 5 illustrates an alternate construction of the actuating elementfor the warning switch.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, ahousing 10 defines a primary bore 12 containing front hydraulic systempiston 14 actuated by pedal 15 through pushrod 17 for energizing frontbrakes 19. A retainer 16 suitably secured to housing 10 is engaged bypiston 14. A rear hydraulic system piston 18 is actuated by fluidpressure in a front system pressure chamber 21 created by movement offront system piston 14, or; if there is no fluid pressure in frontsystem pressure chamber 21, by contact of a front system pistonextension 38 with a shroud 40 for energizing the rear brakes 23. Therear system piston 18 has a secondary bore and an extension 25 with anaxial passageway 22 therethrough communicating with the secondary bore20 and rear system pressure chamber 24. Spool section 26 of piston 18provides an annular cavity 28 around the piston 18 for retaining andopera- .tively carrying expandable sleeve 30 or, in the modification ofFIG. 5, lever 36.

FIG. 2 illustrates the radial passages 32 from the hub of the spoolsection 26 into secondary bore 20, through which radial projections 34of the expandable sleeve 30 (illustrated in FIGS. 1, 2 and 3) or radialprojections 34 of levers 36 (illustrated in FIG. 5) are interposed.

Fail piston 42 is reciprocably located in secondary bore 20 and has acamming surface 44 on a portion of its length for engagement with theradial projections 34. An extension 46 of the fail piston 42 providesfor failure mode positioning of the piston 42, by contact with actuationstud 48 when fluid pressure is lost in the front system pressure chamber21 or, when fluid pressure is lost in the rear system pressure chamber24, by the fluid pressure in front system pressure chamber 21 acting onthe effective area of fail piston extension 46, unopposed by anybalancing force on the other end 50 of fail piston 42. Suitableconventional seals 52 are provided to effect a suitable seal between thevarious pistons and their corresponding bores to prevent intermixture offront and rear brake system fluid and to prevent contamination such asdirt or water from entering the mechanism. Piston return springs 54 and55 urge front and rear system hydraulic pistons, respectively, to theirbase positions when no force is applied to the brake pedal 15. Pistonretainer 56 provides both a stop for axial movement of fail piston 42 tothe right as viewed in FIG. I and a pilot diameter for locating the seal52 between the right end of rear system piston 18 and bore 12.Electrical contact 58 has an inner extension 60, notch 62, an outerextension 64, and a reduced passageway diameter 66. Passageway 66 couldbe produced by a number of methods; for example, a series of axialgrooves converging with the diameter of inner extension 60 at somepredetermined sealing point on the length. The notch 62 operativelyengages the stem 68 of a toggle valve 70. A positioning spring 73 urgestoggle valve 70 toward a closed position. The

outer extension 64 completes an electrical circuit through an indicatorcontact 72 and indicator 74 from battery 76 to electrical ground 77whenever there is pressure failure in either front brakes 19 or rearbrakes 23. Contact spring 78 urges electrical contact 58 toward its baseposition with inner extension 60 contacting unexpanded sleeve 30 orlever 36, passageway 66 open for admission of fluid flow therethroughand notch 62 allowing toggle valve 70 to remain in the closed position.A bypass channel 80, toggle valve passage 82 and electrical contactcavity 84 provide pressure communication from rear system pressurechamber 24 to proportioning valve bypass piston 86. A proportioningvalve 88 is located in a proportioning valve cavity 90 and includespiston 92, a body 94 having a valve seat 95, release spring 96 and apoppet 98. Nut 100 both' anchors poppet 98 in a stationary position andholds the proportioning valve 88 in place.

A metering valve 102 is located in a metering valve cavity 104 but, initself, forms no part of the present in vention. The details of themechanism are more completely described in copending application Ser.No. 62,237, which was filed Aug. 5, 1970 in the name of P. B. Shutt by acommon assignee, and is incorporated herein by reference as necessaryfor a more detailed understanding of the present invention.

MODE OF OPERATION Assuming a normal brake application, the vehicleoperator depresses pedal 15, which, through pushrod 17 transmits a forceto the front hydraulic system piston 14 which upon moving pressurizesthe front system pressure chamber 21. This pressure exerts a force onthe rear hydraulic system piston 18 which then moves to causepressurization of the rear system pressure chamber 24. Since the rearsystem face 50 of the fail piston 42 is larger (i.e., has more effectivearea) than the fail piston extension face 46, a force, represented bythe expression (.4 I A P= F, in which minor frictional force effects areignored, is exerted on the fail piston 42 toward the right as viewed inFIG. 1 tending to hold it in its base or normal unactuated position. Theterms of the expression defining force F are as follows: A is the areaof the rear system face 50 of the fail piston 42; A is the area of thefail piston extension face 46 and P is the fluid pressure in either thefront or rear system pressure chambers. With the fail piston in thisbase position radial projections 34 are located in a depression oncamming surface 44 allowing expandable sleeve 30 or lever 36 to lieagainst the spool section 26 of the rear system piston 18. Electricalcontact 58 having an inner extension 60 slides transversely along theoutside of expandable sleeve 30 or lever 36 during brake application andrelease with no axial motion of the electrical contact 58. Passageway 66is open, notch 62 is centered over toggle valve stem 68 allowing togglevalve 70 to remain closed, and outer extension 64 is not touchingindicator contact 72. The circuit, from battery 76 through indicator 74to ground 77 is therefore open and the failure indicator 74 does notfunction. 1

As more force is exerted on pistons 14 and 18 by the operator throughpedal 15 and pushrod 17, the pressures in front system pressure chamber21 and rear system pressure chamber 24 continue to rise. In the case ofthe rear braking system, this pressureis transmitted directly to therear brakes 23 until a predetermined pressure is reached, indicating abraking intensity such that a considerable portion of the weight of thevehicle has been shifted to the front wheels, whereupon with furtherpressure increase, proportioning valve body 94, urged by the pressure ofthe fluid against the bottom side of proportioning valve piston 92,moves upward as viewed in FIG. 1 against proportioning valve releasespring 96, thus restricting flow through proportioning valve seat 95.The poppet 98 remains stationary. Therefore, a progressively decreasingproportion of the fluid pressure generated in system chamber 24 isallowed to pass to the rear brakes 23. This restriction of the pressureincrease allowed to pass to the rear brakes prevents rear wheel lock-upand the concomitant loss of control over the vehicle as weight of thevehicle is shifted to the front axle during rapid deceleration of thevehicle caused by vigorous brake application.

Metering valve 102 more fully described in the aforementioned copendingapplication Ser. No. 62,237 does not immediately pass on pressure fromfront system chamber 21 to front disc brakes l9..lnstead, at somepredetermined pressure, approximately equal to that required to move theshoes of rear drum brakes 23 into contact with the drum, the pressurefrom front system chamber 21 begins to pass to the front system discbrakes, thus actuating front and rear brakes at approximately the sametime.

If there is fluid loss, or any cause for pressure loss in the fronthydraulic system, front system pressure chamber 21 is not pressurized asthe operator depresses pedal causing movement of pushrod l7 and frontsystem piston 14. Piston 14 moves to the left as viewed in FIG. 1against the front system piston return spring 54 until the fail switchactuation stud 48 encounters fail piston extension 46. As piston 14movement continues to the left, fail piston 42 is shifted to its failposition (i.e., to the left as viewed in FIG. 1) with radial projections34 moved out of the depression of camming surface 44 onto the shouldercreated by the outside diameter of fail piston 42. Expandable sleeve 30or lever 36 is then moved radially outwardly from spool section 26 ofrear system piston 18 into cavity 28. This movement transmitted throughinner extension 60 causes electrical contact 58 to move axiallydownwardly against contact spring 78, causing passageway 66 to close,notch 62 to encounter toggle valve stem 68 thus opening toggle valve 70,and outer extension 64 to touch indicator contact 72, completing thecircuit from battery 76 through indicator 74, indicator contact 76, toelectrical ground 77. Indicator 74 then continuously maintains a failurewarning to the vehicle operator until the malfunctioning system isrepaired and the brakes reapplied. At approximately the same point thefail piston completes its axial travel for failure positioning, frontsystem piston extension 38 encounters shroud 40 causing movement of rearsystem piston 18 to the left as viewed in FIG. 1 and resulting in rearsystem pressure chamber 24 pressurization. This pressure is communicatedthrough proportioning valve 88 to the rear wheels of the vehicle andthrough toggle valve passage 82, past open toggle valve 70 intoelectrical contact cavity 84 and back through bypass channel 80 to thetopside of proportioning valve bypass piston 86 in proportioning valvecavity 90. This pressure in concert with proportioning valve releasespring 96 create a force overcoming that created by the fluid pressureacting on proportioning valve piston 92 so that proportioning valve body94 is held downward, as viewed in FIG. 1, and the proportioning .valveremains open to admit a free flow of pressure therethrough and thusprovides maximum utilization of the remaining rear brakes regardless ofthe pressure generated in rear system chamber 24.

If fluid pressure loss occurs in the rear braking system, rear systemchamber 24 is not pressurized as the operator depresses pedal 15.Instead, fluid pressure generated in front system chamber 21 and actingon the face of fail piston extension 46, unbalanced by any force on rearsystem face 50, causes fail piston 42 to move to the left as viewed inFIG. 1 and, as previously explained, failure of the front braking systemto energize indicator 74 for warning the operator of the brake systemfailure continuously until the cause for malfunction has been repairedand the brakes reapplied.

Piston 18, urged by fluid pressure in front system chamber 21, moves tothe left as viewed in FIG. 1 until rear system piston extension 25encounters the end of bore 12 whereupon the pressure may rise in thefront system pressure chamber 21 according to the pedal 15 depressionforce for actuating the remaining front brakes 19.

Upon repair of the cause of failure in either the front or rear brakingsystems, and a subsequent brake application, fail piston 42 isrepositioned in its normal or base position by equal fluid pressures inchambers 21 and 24 acting as previously explained on the unequal areaopposed piston faces 46 and 50, respectively. The resulting unbalancedforce moves the fail piston 42 to the right as viewed in FIG. 1, thusallowing radial projections 34 to re-enter the depression on cammingsurface 44. This in turn through radially inward motion of sleeve 30 orlever 36 and axially upward motion as viewed in FIG. 1 of electricalcontact 58 cause the indicator 74 to be de-energized and theproportioning valve stem 68 to be freed from the restraint of notch 62,thus allowing positioning spring 73 to return the toggle valve to itsnormally closed position which then reinstates proportioning valve 88functionally as previously explained for the rear brakes 23.

I claim: 1. In a fluid motor for use in a vehicle braking system havingfirst and second hydraulic systems: a housing defining a boretherewithin; pressurizing means for developing fluid pressure withinsaid first and second hydraulic systems, said pressurizing means havingfirst and second pistons in said bore, said first piston beingreciprocable with respect to said second piston;

operator-actuated means operatively connected to said pressurizing meansfor energizing the same;

said bore being divided by said pressurizing means into first and secondhydraulic chambers;

first and second outlet passage means in said housing for communicatingpressurized fluid from said first and second hydraulic chambers,respectively, to front and rear brakes, respectively; pressure delayingmeans operatively connected to said first outlet passage means forregulating pressure transmission from said first hydraulic chamber tosaid front brakes;

fluid pressure responsive pressure proportionin means operativelyconnected to said second outlet passage means for regulating pressuretransmission from said second hydraulic chamber to said rear brakes;

actuating means operatively connected to said pressure proportioningmeans for actuating the same to a predetermined open position;

control means operatively connected to said actuating means andoperative in response to a predetermined pressure loss in said firsthydraulic pressure chamber for energizing said actuating means; warningmeans operatively connected to said control means and adapted to warn avehicle operator of said pressure loss in said front brake, said warningmeans having a second bore within said second piston, said second pistonincluding an extension, with a small diameter axial passagetherethrough, one end of said second piston defining a shoulder at thejuncture of said second bore and said passage;

an external reduced diameter section on said second piston with at leastone radial passage extending inwardly therethrough from said externalreduced diameter section to said second bore;

a gland threadedly engaging said second bore at the other end of saidsecond piston having an internal stepped bore to present a larger and asmaller portion with a shoulder therebetween;

wherein said control means includes, a third piston reciprocable in saidsecond bore having a pair of opposed faces and a camming surfacethereon, each of said first and second hydraulic systems being incommunication with one of said faces whereby a predetermined pressuredifferential between said first and second hydraulic systems will shiftsaid piston;

movable member disposed around said external reduced diameter sectionhaving at least one projection extending radially inwardly through saidradial passage for engagement .with said camming surface; and

wherein said warning means includes electrical contact means abuttingsaid movable member for converting motion of the latter to an electricalsignal when a predetermined pressure differential exists between saidfirst and second hydraulic chambers.

2. The invention of claim 1 wherein:

said pressure proportioning means includes fluid pressure responsiveproportioning valve means responsive to fluid back pressure from saidrear brakes for decreasing the pressure transmission from said secondhydraulic chamber to said rear brakes at a predetermined brakeapplication intensity.

3. The invention of claim 1 wherein said second piston is adapted torelocate said third piston to a predetermined position upon restorationof equal pressures in said first and second chambers.

4. The invention of claim 1 wherein said movable member comprises aradially expanding sleeve.

5. The invention of claim 1 wherein said movable member comprises alever.

6. The invention of claim 4 wherein said radially expanding sleeve isformed of a resilient material and includes a cut through the wall ofsaid sleeve coaxial with said sleeve from one end of said sleeve to theother for said housing having a second bore with a conical seat on theapex of said second bore and an annular piston at its outer peripheryslidably received in said outlet passage of said second hydraulicchamber;

means for affixing a stationary poppet within said outlet passage havinga circular conical head for mating with said conical seat; and

resilient means located in said outlet passage urging said annularpiston toward an open valve position.

9. In a fluid motor for use in a vehicle braking system having first andsecond hydraulic systems:

a housing defining a bore therewithin;

pressurizing means for developing fluid pressure within said first andsecond hydraulic system;

operator-actuated means operatively connected to said pressurizing meansfor energizing the same;

said bore being divided by said pressurizing means into first and secondhydraulic chambers;

first and second outlet passage means in said housing for communicatingpressurized fluid from said first and second hydraulic chambers,respectively, to front and rear brakes, respectively;

pressure delaying means operatively connected to said first outletpassage means for regulating pressure transmission from said firsthydraulic chamber to said front brakes;

fluid pressure responsive pressure proportioning means operativelyconnected to said second outlet passage means for regulating pressuretransmission from said second hydraulic chamber to said rear brakes;

actuating means operatively connected to said pressure proportioningmeans for actuating the same to a predetermined open position, saidactuating means having a fluid pressure responsive means;

control means operatively connected to said actuating means andoperative in response to a predetermined pressure loss in said firsthydraulic pressure chamber for energizing said actuating means, saidcontrol means having a source of pressurized fluid and valve means inflow control relationship therewith for controlling pressurization ofsaid fluid pressure responsive means; and

warning means operatively connected to said control means and adapted towarn a vehicle operator of said pressure loss in said front brakingsystem.

10. The invention of claim 9 wherein said valve means is operativelyconnected to said warning switch means so that said valve means and saidwarning switch means are simultaneously actuated in response to saidpressure loss.

11. The invention of claim 10 wherein said operative connection betweensaid valve means and said warning switch means includes electricalcontact means.

12. The invention of claim 10 wherein said valve means includes a valveseat and a tilt valve shiftable from said seat to establish fluidcommunication between said source and said fluid pressure responsivemeans to actuate said pressure proportioning means to said predeterminedopen position thereby permitting substantially unrestricted pressuretransmission from said second chamber to said rear brakes upon pressurefailure in said front braking system.

operably engaging said valve means.

14. The invention of claim 12 wherein said tilt valve is biased.

1. In a fluid motor for use in a vehicle braking system having first andsecond hydraulic systems: a housing defining a bore therewithin;pressurizing means for developing fluid pressure within said first andsecond hydraulic systems, said pressurizing means having first andsecond pistons in said bore, said first piston being reciprocable withrespect to said second piston; operator-actuated means operativelyconnected to said pressurizing means for energizing the same; said borebeing divided by said pressurizing means into first and second hydraulicchambers; first and second outlet passage means in said housing forcommunicating pressurized fluid from said first and second hydraulicchambers, respectively, to front and rear brakes, respectively; pressuredelaying means operatively connected to said first outlet passage meansfor regulating pressure transmission from said first hydraulic chamberto said front brakes; fluid pressure responsive pressure proportioningmeans operatively connected to said second outlet passage means forregulating pressure transmission from said second hydraulic chamber tosaid rear brakes; actuating means operatively connected to said pressureproportioning means for actuating the same to a predetermined openposition; control means operatively connected to said actuating meansand operative in response to a predetermined pressure loss in said firsthydraulic pressure chamber for energizing said actuating means; warningmeans operatively connected to said control means and adapted to warn avehicle operator of said pressure loss in said front brake, said warningmeans having a second bore within said second piston, said second pistonincluding an extension, with a small diameter axial passagetherethrough, one end of said second piston defining a shoulder at thejuncture of said second bore and said passage; an external reduceddiameter section on said second piston with at least one radial passageextending inwardly therethrough from said external reduced diametersection to said second bore; a gland threadedly engaging said secondbore at the other end of said second piston having an internal steppedbore to present a larger and a smaller portion with a shouldertherebetween; wherein said control means includes, a third pistonreciprocable in said second bore having a pair of opposed faces and acamming surface thereon, each of said first and second hydraulic systemsbeing in communication with one of said faces whereby a predeterminedpressure differential between said first and second hydraulic systemswill shift said piston; a movable member disposed around said externalreduced diameter section having at least one projection extendingradially inwardly through said radial passage for engagement with saidcamming surface; and wherein said warning means includes electricalcontact means abutting said movable member for converting motion of thelatter to an electrical signal when a predetermined pressuredifferential exists between said first and second hydraulic chambers. 2.The invention of claim 1 wherein: said pressure proportioning meansincludes fluid pressure responsive proportioning valve means responsiveto fluid back pressure from said rear brakes for decreasing the pressuretransmission from said second hydraulic chamber to said rear brakes at apredetermined brake application intensity.
 3. The invention of claim 1wherein said second piston is adapted to relocate said third piston to apredetermined position upon restoration of equal pressures in said firstand second chambers.
 4. The invention of claim 1 wherein said movablemember comprises a radially expanding sleeve.
 5. The invention of claim1 wherein said movable member Comprises a lever.
 6. The invention ofclaim 4 wherein said radially expanding sleeve is formed of a resilientmaterial and includes a cut through the wall of said sleeve coaxial withsaid sleeve from one end of said sleeve to the other for allowingexpansion of said sleeve.
 7. The invention of claim 4 wherein saidradially expandable sleeve is formed with a series of axial cutsextending over the major portion of the length of said sleeve, eachsuccessive axial cut through the wall of said sleeve alternativelyextending to one end of said sleeve and the next to the other end ofsaid sleeve.
 8. The invention of claim 2 wherein: said housing having asecond bore with a conical seat on the apex of said second bore and anannular piston at its outer periphery slidably received in said outletpassage of said second hydraulic chamber; means for affixing astationary poppet within said outlet passage having a circular conicalhead for mating with said conical seat; and resilient means located insaid outlet passage urging said annular piston toward an open valveposition.
 9. In a fluid motor for use in a vehicle braking system havingfirst and second hydraulic systems: a housing defining a boretherewithin; pressurizing means for developing fluid pressure withinsaid first and second hydraulic system; operator-actuated meansoperatively connected to said pressurizing means for energizing thesame; said bore being divided by said pressurizing means into first andsecond hydraulic chambers; first and second outlet passage means in saidhousing for communicating pressurized fluid from said first and secondhydraulic chambers, respectively, to front and rear brakes,respectively; pressure delaying means operatively connected to saidfirst outlet passage means for regulating pressure transmission fromsaid first hydraulic chamber to said front brakes; fluid pressureresponsive pressure proportioning means operatively connected to saidsecond outlet passage means for regulating pressure transmission fromsaid second hydraulic chamber to said rear brakes; actuating meansoperatively connected to said pressure proportioning means for actuatingthe same to a predetermined open position, said actuating means having afluid pressure responsive means; control means operatively connected tosaid actuating means and operative in response to a predeterminedpressure loss in said first hydraulic pressure chamber for energizingsaid actuating means, said control means having a source of pressurizedfluid and valve means in flow control relationship therewith forcontrolling pressurization of said fluid pressure responsive means; andwarning means operatively connected to said control means and adapted towarn a vehicle operator of said pressure loss in said front brakingsystem.
 10. The invention of claim 9 wherein said valve means isoperatively connected to said warning switch means so that said valvemeans and said warning switch means are simultaneously actuated inresponse to said pressure loss.
 11. The invention of claim 10 whereinsaid operative connection between said valve means and said warningswitch means includes electrical contact means.
 12. The invention ofclaim 10 wherein said valve means includes a valve seat and a tilt valveshiftable from said seat to establish fluid communication between saidsource and said fluid pressure responsive means to actuate said pressureproportioning means to said predetermined open position therebypermitting substantially unrestricted pressure transmission from saidsecond chamber to said rear brakes upon pressure failure in said frontbraking system.
 13. The invention of claim 11 wherein said electricalcontact means has a depression located thereon for operably engagingsaid valve means.
 14. The invention of claim 12 wherein said tilt valveis biased toward said seat by resilient means.